Pretreatment of a phosphorus-modified zeolite catalyst for an aromatic alkylation process

ABSTRACT

This invention relates to a process for pretreating a zeolite catalyst, specifically a zeolite which has been modified with phosphorus. The catalyst may be used in a process for alkylation of aromatics, specifically toluene methylation. The pretreatment is first to contact the catalyst with the process reactants used in a process for alkylation of aromatics for at least two hours at conditions to produce an alkylated aromatic product and then with a gaseous stream containing oxygen at a temperature and for a time until there is no oxygen consumption. The zeolite may be a MFI zeolite. This pretreatment procedure for a phosphorus-modified zeolite catalyst produces a catalyst which has increased run time, i.e., decreased deactivation rate, compared to a fresh catalyst, even after successive regenerations.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/080,354 filed Apr. 2, 2008, which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for alkylation of aromatics, e.g.,toluene methylation, with a zeolite catalyst, e.g., an aluminosilicatezeolite which has been modified with phosphorus, said process includingpretreatment of the phosphorus-modified zeolite catalyst.

2. Description of the Prior Art

Zeolites are crystalline solids made up of aluminum-substituted SiO₄tetrahedral units joined together to form different ring and cagestructures into a crystalline framework. The physical structure ofzeolite is very porous with a large internal and external surface area.The substitution of aluminum generates a charge imbalance which must becountered by a supplementary counterion, such as a proton.

Zeolites can be shape-selective catalysts due to steric and electroniceffects. Selective reactions can occur over zeolites as certainproducts, reactants or transition states are kept from forming withinthe pores either by transition state selectivity or because of size orshape of molecular diameter. By varying the preparation of zeolitecatalysts, they can be modified to carry out very specific syntheses ofdesired products. Elements may be deposited on the zeolite to enhanceproperties of a zeolite catalyst used in particular processes.

Modified zeolite catalysts are known for alkylation of aromatics,specifically methylation of toluene to xylenes, especially p-xylene

Toluene methylation (TM) is a catalytic reaction of toluene withmethanol to produce xylenes as shown below:

All of these xylene isomers, meta-xylene (m-xylene), ortho-xylene(o-xylene) and para-xylene (p-xylene), are important chemicalintermediates. o-Xylene is oxidized to make phthalic anhydride which isused to make phthalate plasticizers among other things. m-Xylene isoxidized to make isophthalic acid, which is used in unsaturatedpolyester resins (UPR). However, p-xylene has by far the largest marketof the three isomers. The largest use of p-xylene is in its oxidation tomake terephthalic acid. Terephthalic acid is used in turn to makepolymers such as polyethylene terephthalate (PET) and polybutyleneterephthalate (PBT). PET is one of the largest volume polymers in theworld. As such the demand for p-xylene is several times that for m- ando-xylene. In commercial manufacture p-xylene is purified from mixedxylenes by crystallization and adsorption processes.

Thermodynamic equilibrium compositions of o-, m-, and p-xylenes areapproximately 25, 50 and 25%, respectively, at 500° C. The catalyticprocesses such as toluene disproportionation (TDP) and TM would giveabout 25% p-xylene (PX) in mixed-xylenes (MX). However, if a catalystpossesses shape selective properties it will give significantly greaterthan 25% PX. Typically, a shape selective catalyst would give >85% PX inMX.

Zeolites as catalysts for isomerization, toluene disproportionation,transalkylation, hydrogenation and alkane oligomerization andaromatization deactivate over time when operated under commercialprocess conditions and must be regenerated to continue to be used in areaction system. In zeolite catalysts deactivation is at least in partattributed to the formation of carbonaceous deposits; i.e., “coke”, onthe active site of the catalyst. Once the coking has affected catalystperformance to the point that product yield is no longer advantageous,the coke must be burned off of the catalyst in an oxygen-containingenvironment. The regenerated catalyst is then reintroduced into contactwith the reactants and run until such time as coking again reducesperformance to an unacceptable level. The catalyst is again regeneratedand this cycle repeats.

U.S. Pat. No. 4,456,780 discloses a method for pretreatment of a zeolitecatalyst with steam and/or a phosphorus-containing compound to decreasecoking and extend catalyst life. Pretreatment may be by depositingapproximately 4% by weight phosphorus, by contacting the catalyst withsteam at 250-1000° C. for 15 minutes to 100 hours or by a combination ofphosphorus deposition and steaming.

U.S. Pat. No. 6,504,072 discloses a process for the selective productionof para-xylene by reacting toluene with methanol in the presence of acatalyst of a medium-pore zeolite, such as ZSM-5, which has beencombined with an oxide modifier, such as phosphorus, and severelysteamed at a temperature of at least 950° C. The catalyst may beregenerated after accumulating coke in the toluene methylation reactionby burning off a controlled amount of coke in a partial combustionatmosphere at temperature in the range of from about 400 to about 700°C.

U.S. Pat. No. 3,965,208 discloses a process for the methylation oftoluene in the presence of a catalyst of zeolite modified with a GroupVA element, such as phosphorus, antimony and arsenic, in an amount of atleast 0.5% by weight. The catalyst is regenerated by burning coke fromthe catalyst in an oxygen-containing atmosphere, such as air, atelevated temperatures. The catalyst is reactivated by passing avaporized Group VA compound through the catalyst bed. One example ofreactivation is to pass an equal volume mixture of toluene and diphenylphosphine chloride at a temperature of about 250° C. for about one-halfhour and then heating in air at 150 cc/minute at about 550° C. for aboutone-half hour.

It is desirable to extend the process run time between regenerationcycles.

SUMMARY OF THE INVENTION

A phosphorus-modified zeolite catalyst is used in a process for thealkylation of aromatics comprising:

a) pretreating the phosphorus-modified zeolite catalyst by:

-   -   1) first contacting the catalyst with alkylation process        reactants for at least two hours at process conditions to        produce an alkylated aromatic product, and    -   2) second by contacting the catalyst with a gaseous stream        comprising oxygen at a temperature and for a time until there is        no oxygen consumption;

b) contacting the pretreated catalyst with alkylation process reactantsat process conditions to produce an alkylated aromatic product; and

c) recovering an alkylated aromatic product.

It has been found that pretreating a phosphorus-modified zeolitecatalyst in such a manner produces a catalyst which has increased runtime, i.e., decreased deactivation rate, compared to a fresh catalyst.The process may further comprise:

d) regenerating the catalyst;

e) contacting the regenerated catalyst with alkylation process reactantsat process conditions to produce an alkylated aromatic product; and

f) repeating steps d) and e).

The zeolite may be MFI aluminosilicate, e.g., phosphorus-modified ZSM-5.A pretreated catalyst has toluene conversion and p-xylene selectivitywhich are both greater than 95% of that for the original fresh catalyst,even after successive regenerations.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily understood by reference to thefollowing detailed description when considered in connection with theaccompanying drawings:

FIG. 1 is a schematic diagram of a toluene methylation reaction unitwith reactors in series

FIG. 2 is a graph of time on stream in days v. toluene conversion forExample 1

FIG. 3 is a graph of time on stream in days v. toluene conversion forExample 2

FIG. 4 is a graph of time on stream in days v. toluene conversion forExample 3

DETAILED DESCRIPTION OF THE INVENTION

Toluene methylation is known to occur over zeolite or zeolite-typecatalysts, in particular, ZSM-5-type zeolite catalysts. Generally, athermodynamic equilibrium mixture of ortho (o)-, meta (m)- and para(p)-xylenes is formed from the methylation of toluene. Thermodynamicequilibrium compositions of o-, m-, and p-xylenes may be around 25, 50and 25 mole %, respectively, at a reaction temperature of about 500° C.Such toluene methylation may occur over at wide range of temperatures,however.

A high purity grade (99+%) p-xylene is desirable for its oxidation toterephthalic acid process. Thus, an increased concentration of p-xyleneover equilibrium is desirable. However, production cost for such aconcentration can be very high. p-Xylene can be separated from mixedxylenes by cycle of adsorption and isomerization which must be repeatedmany times because of its low isomeric concentration in the equilibriummixture. If the concentration of p-xylene is higher than equilibrium,the high purity grade p-xylene can be more easily attained. An amount ofp-xylene significantly higher than equilibrium can be obtained if thecatalyst contains shape selective properties. Such shape selectiveproperties can be incorporated in zeolite catalyst by modifying thezeolite.

Zeolite is a crystalline hydrated aluminosilicate that may also containother metals, such as sodium, calcium, barium, and potassium, and thathas ion exchange properties (Encarta® World English Dictionary [NorthAmerican Edition] © & (P) 2001 Microsoft Corporation). Examples ofzeolites are ZSM-5, ZSM-11, ZSM-5/ZSM-11 intermediate, ZSM-12, ZSM-21,ZSM-22, ZSM-23, ZSM-35, ZSM-38, ZSM-48, ZSM-50, MCM-22, Zeolite L,Zeolite Beta and Mordenite which are known in the art.

ZSM-5 zeolite is a porous material containing intersectingtwo-dimensional pore structure with 10-membered oxygen rings. Zeoliteswith such 10-membered oxygen ring pore structures are often classifiedas medium-pore zeolites. As used herein, the expression “ZSM-5-type” ismeant to refer to those zeolites that are isostructurally the same asZSM-5 zeolites. Additionally, the expressions “ZSM-5” and “ZSM-5-type”may also be used herein interchangeably to encompass one another andshould not be construed in a limiting sense.

ZSM-5 zeolite catalysts and their preparation are described in U.S. Pat.No. 3,702,886, which is hereby incorporated by reference. In the presentinvention, the ZSM-5 zeolite catalyst may include those having asilica:alumina molar ratio of at least about 25, of 200 or higher orfrom about 250 to about 1000 prior to modification. The starting ZSM-5may be an NH₄ ⁺ or H⁺ form and may contain traces of other cations.

Modification of ZSM-5-type zeolite catalysts with phosphorus-containingcompounds has been shown to provide shape selective properties to thecatalyst, yielding significantly greater amounts of p-xylene than thethermodynamic equilibrium value when used in toluene methylationcompared to unmodified catalysts. Such modification has been shown toprovide selectivity for p-xylenes of greater than 80%.

The ZSM-5 may be modified by treating with phosphorus-containingcompounds including, but are not limited to, phosphonic, phosphinous,phosphorus and phosphoric acids, salts and esters of such acids andphosphorous halides. In particular, phosphoric acid (H₃PO₄) and ammoniumhydrogen phosphate ((NH₄)₂HPO₄) may be used as the phosphorus-containingcompound to provide a catalyst for toluene methylation with shapeselective properties to provide increased p-xylene selectivity. Suchmodified catalysts may contain phosphorus (P) in an amount of from about0.01 to about 0.15 g P/g zeolite, more particularly from about 0.02 toabout 0.13 g P/g zeolite, and more particularly from about 0.07 g P/gzeolite to about 0.12 g P/g zeolite, and still more particularly fromabout 0.09 g P/g zeolite to about 0.11 g P/g zeolite. After phosphorustreatment, the phosphorus-treated zeolite may be dried.

The phosphorus-modified ZSM-5 may be made by forming a slurry of aZSM-5-type zeolite and an aqueous solution of a phosphorus compound andremoving water from the slurry to form a phosphorus-modified ZSM-5zeolite. The phosphorus-modified catalyst prepared as described in U.S.Pat. No. 7,285,511 issued Oct. 23, 2007, which is hereby incorporated byreference, is not steamed and has a pore volume of from 0.2 ml/g orless.

The phosphorus-modified ZSM-5 may be made by dissolving alumina in aphosphorus-containing acid solution and treating the zeolite with thedissolved alumina solution as described in U.S. Pat. No. 6,943,131issued Sep. 13, 2005, which is hereby incorporated by reference.

The phosphorus-modified ZSM-5 may have particular ³¹P MAS NMR peaksindicating the present of free phosphate, phosphate bonded toextra-framework aluminum, or particular phosphate species as describedin Published U.S. Patent Application no. 2005-0240070 A1 published Oct.27, 2005, which is hereby incorporated by reference. The catalyst mayexhibit at least two ³¹P MAS NMR peaks having maxima at from about 0 ppmto about −55 ppm. More particularly, the catalyst may exhibit a ³¹P MASNMR peak having a maximum at from about 0 ppm to about −25 ppm, moreparticularly at from about −5 ppm to about −20 ppm, and another with amaximum at from about −40 ppm to about −50 ppm. Such peaks are anindication of various phosphorus species.

Zeolites other than ZSM-5 which are useful in the present invention aremedium pore zeolites that have 10 and/or 12 member ring channels system,such as ZSM-4 (Zeolite Omega), ZSM-11, ZSM-12, ZSM-22, ZSM-23, ZeoliteBeta, Mordenite, MCM-22 and combinations and mixtures thereof.Silica-alumina phosphates (SAPO), aluminum phosphates (AlPO) andcombinations and mixtures thereof are also useful in the presentinvention.

The phosphorus-modified zeolite may be heated at 300° C. or higher afterphosphorus treatment and then combined with an inorganic oxide bindermaterial to form a zeolite-binder mixture which forms a bound zeolitecatalyst as described in Published U.S. Patent Application no.2007-0032690 A1 published Feb. 8, 2007, which is hereby incorporated byreference.

The phosphorus-modified zeolite catalyst can be combined with aninorganic oxide binder material which has been treated with a mineralacid to form a zeolite-binder mixture and heating the zeolite-bindermixture at temperature of about 400° C. or higher to form a boundzeolite catalyst as described in Published U.S. Patent Application No.2007-0149384 A1 published Jun. 28, 2007, which is hereby incorporated byreference.

The bound P-modified zeolite catalyst may be mildly steamed at atemperature of 300° C. or lower before using the catalyst in anyreaction. The steaming can be carried out in-situ or ex-situ of thereactor. The use of catalyst steaming at mild temperatures is describedin co-pending U.S. Pat. No. 7,304,194 issued Dec. 4, 2007, entitled“Hydrothermal Treatment of Phosphorus-Modified Zeolite Catalysts,” whichis herein incorporated by reference.

The P-modified ZSM-5 catalyst may be contacted with an appropriate feedof alkylation process reactants, such as an aromatic hydrocarbon and analkylating agent, under process conditions to produce an alkylatedaromatic product. The catalyst has particular application for use intoluene methylation utilizing a toluene/methanol feed. As used herein,the expression “alkylation process reactants” is meant to encompass thearomatic compound and the alkylating agent which include toluene andmethanol, respectively.

A gas cofeed may also be used with the alkylation process reactants. Thecofeed gas may include hydrogen or an inert gas. In addition to anycofeed gas, water that may be in the form of steam may also beintroduced into the reactor as cofeed along with the alkylation feed.The water or steam used for the methylation reaction may be introducedwith or without hydrogen or inert gas as cofeed with the alkylation feedto the reactor during the start up of the alkylation reaction, or it maybe introduced subsequent to initial start up. In either case, liquidwater may be added and vaporized prior to its mixing with cofeed gas (ifany) and the alkylation feed. The use of water cofeed is described inU.S. Pat. No. 7,060,864 issued Jun. 13, 2006, entitled “TolueneMethylation Process,” and in U.S. Pat. No. 7,279,608 issued Oct. 9,2007, entitled “Toluene Methylation Process with Increased MethanolSelectivity”, both of which are herein incorporated by reference.

The reactor pressure for toluene methylation or other aromaticalkylation may vary, but typically ranges from about 10 to about 1000psig. Reactor temperatures may vary, but typically range from about 400to about 700° C. In one embodiment of the invention, reactor pressure isfrom about 20 psig to about 50 psig and reactor temperature is fromabout 500° C. to about 600° C. Upon introduction of feed into thereactor, the catalyst bed temperature may be adjusted to a selectedreaction temperature to effect a desired conversion.

The temperature may be increased gradually at a rate of from about 1°C./min to about 10° C./min to provide the desired final reactortemperature. As used in the examples, reactor temperature refers to thetemperature as measured as an average temperature of catalyst bed of thereactor. The zeolite catalyst may be regenerated by contacting thecatalyst with an oxygen-containing gas until no oxygen consumption isdetected, e.g., at a temperature of about 450° C. to about 700° C. forabout one to about twenty hours. One means of oxygen detection is withan oxygen analyzer, such as a paramagnetic oxygen sensor with a lowerdetection limit of 0.01% by volume. By the term “no oxygen consumptionis detected” it is meant to be a level of not more than 0.01% by volume.In one embodiment of the invention, the catalyst is regenerated at atemperature of about 500° C. to about 600° C. for about twelve to aboutfifteen hours.

The catalyst of the claimed invention may be used in a process fortoluene methylation with a startup procedure in which thetoluene/methanol feed is introduced into the reactor at a relativelyhigh liquid hourly space velocity (LHSV) with a cofeed of hydrogen forone-half to about 20 hours before running the reactor at a relativelylower LHSV as described in U.S. Pat. No. 7,084,318 issued Aug. 1, 2006,which is herein incorporated by reference.

The reaction may be carried out in a variety of different reactors thatare commonly used for carrying out aromatic alkylation reactions. Singleor multiple reactors in series and/or parallel are suitable for carryingout the aromatic alkylation. Methanol and/or toluene may be added to theproduct stream entering the second and subsequent reactors when usingmultiple reactors in series.

The P-modified ZSM-5 zeolite catalyst, as described herein, hasparticular application for use in toluene methylation for preparing axylene product from a feed of toluene and methanol. The catalystprovides increased selectivity for p-xylene when used in toluenemethylation. In particular, the catalyst may provide greater than 85%,90% or 95% para-xylene selectivity when used in toluene methylation.Additionally, in certain instances, greater than 95% of total xyleneselectivity may be achieved.

Processes useful for the present invention other than toluenemethylation would include aromatic alkylation and transalkylation,toluene disproportionation, methanol to gasoline (MTG) processes andn-paraffin (C₆ and higher) cyclization.

The P-modified zeolite catalyst is pretreated by first contacting thecatalyst with a feed containing toluene, methanol, hydrogen, water underconditions for a toluene methylation reaction. Alkylated aromaticproduct may be made during the pretreatment procedure. Contact time isat least about two hours or greater. In one embodiment of the invention,reactor temperature is from about 500° to about 600°. Carbonaceousmaterial (coke) may be deposited on the catalyst during this reaction.Contact times may range from about three hours to about ten days but maybe as long as twenty days or one hundred days, depending on the reactionconditions. Contact time may be determined by decline in catalystactivity which may be 50% or less. The carbon (coke) content of thecatalyst may be from about 0.5% to about 20% by weight after this firstcontact in the pretreatment. The catalyst is further pretreated bycontacting the catalyst with oxygen until no oxygen consumption isdetected, e.g., at a temperature of about 450° C. to about 700° C. forat least two hours. Oxygen may be mixed with other gases, e.g., inertgases, such as nitrogen or steam. The gas flow may be at a flow rate ofabout one to about ten weight hourly space velocity (WHSV=h⁻¹).Catalysts pretreated as above have been found to have improveddeactivation rates for aromatic alkylation. The term “deactivation rate’will be used to mean activity decline measured as mole % decrease intoluene conversion during a 24 hour period (mole %/day).

The invention having been generally described, the following examplesare given as particular embodiments of the invention to illustrate, butnot to limit, the invention and to demonstrate the practice andadvantages thereof. It is understood that the examples are given by wayof illustration and are not intended to limit the specification or theclaims to follow in any manner.

As used herein, catalytic activity can be expressed as the % moles ofthe toluene converted with respect to the moles of toluene fed and canbe defined by the following formulas:

Mole % Toluene Conversion=[(T _(i) −T _(o))/T _(i)]×100  (1)

where, T_(i) is the number of moles of toluene fed and T_(o) is thenumber of moles toluene unreacted.

As used herein, selectivity for mixed xylenes may be expressed as:

Mole % Mixed Xylene Selectivity=[X _(tx)/(T _(i) −T _(o))]×100  (2)

where, X_(tx) is the number of moles of mixed (o-, m- or p-) xylenes inthe product.

As used herein, selectivity for p-xylene may be expressed as:

Mole % p-Xylene Selectivity=(X _(p) /X _(tx))×100  (3)

where, X_(p) is the number of moles of p-xylene.

As used herein, methanol conversion may be expressed as:

Mole % Methanol Conversion=[(M _(i) −M _(o))/M _(i)]×100  (4)

where, M_(i) is the number of moles of methanol fed and M_(o) is thenumber of moles methanol unreacted.

As used herein, methanol selectivity for toluene methylation may beexpressed as:

Mole % Methanol Selectivity=[X _(tx)/(M _(i) −M _(o))]×100  (5)

where, X_(tx) is the number of moles of mixed (o-, m- or p-) xylenes,M_(i) is the number of moles of methanol fed and M_(o) is the number ofmoles of unreacted methanol.

Catalyst Preparation

P-modified ZSM-5 catalysts were prepared using NH₄-ZSM-5 zeolite powderhaving SiO₂/Al₂O₃ mole ratio greater than 250 by treating withP-containing compound and then heating to a maximum temperature of about550° C. to form a P/ZSM-5 zeolite powder catalyst. The catalysts werebound with 20% alumina as binder and extruded to make 1/16-inchcylindrical shape extruded catalyst. The extruded catalysts werecalcined or heated at a maximum temperature of about 550° C. for aboutfive hours. Analyses of two batches of the catalysts are shown in Table1.

TABLE 1 Elemental Analysis, wt% N₂ Adsorption Catalyst Na₂O SiO₂ Al₂O₃ PSA, m²/g PV, cc/g Powder Catalyst A <0.04 79.2 0.5 9.4 211 0.13 ExtrudedCatalyst A <0.04 63.4 20.4 7.5 243 0.17 Powder Catalyst B <0.04 76.5 0.610.0 185 0.11 Extruded Catalyst B <0.04 61.0 20.2 7.4 200 0.19

Example 1 Comparative

Extruded Catalyst A was used in a toluene methylation reaction with areactor unit containing three reactors in series (see FIG. 1). In eachof the reactors 40 g of 1/16-inch extruded catalyst was loaded. Thecatalyst was dried at 200° C. under hydrogen flow (1500 sccm) for atleast one hour. The catalyst was steamed by introducing water vapor (6.5g/h) with a carrier gas of H₂ (1500 sccm) at 200° C. overnight. Reactionfeed was introduced as follows: hydrogen rate at 3000 sccm, toluene at90 g/h, steam at 18.5 g/h, and methanol at 6.8 g/h to the 1^(st)reactor. Additional methanol feed was added to 2^(nd) and 3^(rd)reactors at 5.9 g/h and 4.8 g/h, respectively. The inlet pressure forall three reactors was adjusted to 20 psig. Catalyst bed temperature wasadjusted to 530° C., 541° C. and 551° C. for 1^(st), 2^(nd) and 3^(rd)reactor, respectively.

Toluene conversion is shown in FIG. 2. In Table 2 included are tolueneand methanol conversions, mixed-xylene, p-xylene and methanolselectivities. The activity as measured by toluene conversion was foundto decline with time on stream. Toluene conversion was at about 37.3mole % on day #3 and decreased to 35.2 mole % on day #13 with a declinerate of −0.21 mole % per day.

TABLE 2 Time on Conversion, mole % Selectivity, mole% stream, dayToluene Methanol Mixed-Xylene p-Xylene Methanol 2 36.3 94.9 91.3 92.260.7 3 37.3 95.2 93.2 92.3 63.0 4 37.2 94.7 93.2 92.4 63.3 5 36.9 94.293.2 92.6 63.3 6 36.8 93.7 93.4 92.7 63.7 7 36.1 93.5 93.8 92.8 62.8 836.4 93.3 93.6 92.9 63.4 9 36.0 92.9 93.8 93.0 63..1 10 35.8 93.0 94.093.1 62.8 11 35.8 92.8 93.7 93.2 62.7 12 35.4 92.6 94.0 93.3 62.5 1335.2 92.6 94.0 93.4 62.0

Example 2 Catalyst Pretreatment

In this example, the catalyst and the conditions for catalyst loading,drying, steaming, and feed introduction to the reactors were the same asdescribed in Example 1. The toluene methylation reaction continued for 3h at which time the toluene conversion was about 32.7 mole %. Thecatalyst was determined to have a carbon content (coke) of 0.5% byweight. The methylation reaction was stopped after 3 hours of run, allfeeds were stopped, and the catalysts in all three reactors were exposedto controlled flow of oxygen (75 sccm) and nitrogen (1400 sccm). Thecatalyst in all three reactors was treated in the oxygen environment ata maximum temperature of about 600° C. The consumption of oxygen wasmeasured with a California Analytical Model 602P paramagnetic oxygensensor with a lower detection limit for oxygen of 0.01% by volume. Thepretreatment of catalyst under oxygen and nitrogen flow continued for 12h at which time no oxygen consumption was detected.

The methylation reactants (toluene, methanol) and cofeeds (steam, H₂)were reintroduced into the reactors as described earlier. The catalystbed temperature was adjusted to 530° C., 541° C. and 551° C. for 1^(st),2^(nd) and 3^(rd) reactor, respectively. Toluene conversions are shownin FIG. 3. In Table 3 included are toluene and methanol conversions,mixed-xylene, p-xylene and methanol selectivities. In the example,toluene conversion was about 31.9 mole % on day #3, 33.0 mole % on day#4 and 32.2 mole % on day #13. Compared to initial toluene conversion(32.7 mole % as noted above) the catalyst showed improved tolueneconversion with a deactivation rate of −0.04 mole % per day (compared to−0.21 mole % per day for Example 1).

TABLE 3 Time on Conversion, mole % Selectivity, mole % stream, dayToluene Methanol Mixed-Xylene p-Xylene Methanol 1^(a) 32.7 98.7 95.591.8 56.2 3 31.9 97.3 95.6 92.1 55.1 4 33.0 94.9 95.0 92.4 58.2 5 32.893.7 95.2 92.6 59.1 6 32.8 93.2 95.0 92.6 59.3 7 32.5 93.0 95.0 92.759.0 8 32.5 92.4 95.0 92.4 58.7 9 32.7 91.9 95.0 92.9 59.4 10 32.4 91.595.0 93.0 59.5 11 32.4 91.2 94.9 93.1 59.5 12 32.5 90.9 94.8 93.1 59.413 32.6 90.8 94.8 93.1 59.6 ^(a)Before catalyst pretreatment

Example 3 Catalyst Pretreatment

In this example, to determine catalyst stability in the first reactor,the catalyst and the conditions for catalyst loading, drying, steaming,and feed introduction to the reactors were the same as described inExample 1, except after introduction of the feed, catalyst bedtemperature of the first reactor was adjusted to 520° C. The toluenemethylation reaction continued for 13 days during which time the tolueneconversion was found to decrease at −0.0524 mole % per day. Themethylation reaction was stopped after 13 days, all feeds were stopped,and the catalyst in reactor was exposed to controlled flow of oxygen (75sccm) and nitrogen (1400 sccm). The catalyst bed temperature was 585° C.The consumption of oxygen was measured. The pretreatment of catalystunder oxygen and nitrogen flow continued for 12 h at which time nooxygen consumption was detected.

The methylation reactants (toluene, methanol) and cofeeds (steam, H₂)were reintroduced into the reactor as described earlier. The catalystbed temperature was adjusted to 520° C. The methylation reactioncontinued until the day #37. Toluene conversions are shown in FIG. 4. InTable 4 included are toluene and methanol conversions, mixed-xylene,p-xylene and methanol selectivities for the first reactor. In theexample, the rate of decrease in toluene conversion was found to improvewhen the catalyst was pretreated under oxygen environment (−0.0351 mole% per day vs. −0.0524 mole % per day for the fresh load catalyst).

The methylation reaction was stopped on day #37, all feeds were stopped,and the catalyst in reactor was exposed to controlled flow of oxygen (75sccm) and nitrogen (1400 sccm). The catalyst bed temperature was 585° C.The consumption of oxygen was measured. The pretreatment of catalystwith under oxygen and nitrogen flow continued for 12 h at which time nooxygen consumption was detected. The methylation reactants (toluene,methanol) and cofeeds (steam, H2) were reintroduced into the reactor asdescribed earlier. The catalyst bed temperature was adjusted to 520° C.The methylation reaction continued until the day #57. Tolueneconversions are shown in FIG. 4. In Table 4 included are toluene andmethanol conversions, mixed-xylene, p-xylene and methanol selectivitiesfor the first reactor. In the example, the rate of decrease in tolueneconversion was found to improve further when the catalyst was pretreatedand subsequently regenerated under oxygen environment (−0.0081 mole %per day vs. −0.0524 mole % per day for fresh catalyst).

The methylation reaction was stopped on day #57, all feeds were stopped,and the catalyst in reactor was exposed to controlled flow of oxygen (75sccm) and nitrogen (1400 sccm). The catalyst bed temperature was 585° C.The consumption of oxygen was measured. The pretreatment of catalystwith under oxygen and nitrogen flow continued for 12 h at which time nooxygen consumption was detected. The methylation reactants (toluene,methanol) and cofeeds (steam, H₂) were reintroduced into the reactor asdescribed earlier. The catalyst bed temperature was adjusted to 520° C.The methylation reaction continued until the day #76. Tolueneconversions are shown in FIG. 4. In Table 4 included are toluene andmethanol conversions, mixed-xylene, p-xylene and methanol selectivitiesfor the first reactor. In the example, the rate of decrease in tolueneconversion was found to improve further when the catalyst was pretreatedunder oxygen environment (−0.0013 mole % per day vs. −0.0524 mole % perday for fresh load catalyst). Catalyst deactivation is summarized inTable 5.

TABLE 4 Time on stream, Conversion, mole % Selectivity, mole % dayToluene Methanol Mixed-Xylene p-Xylene Methanol 2 13.9 94.3 63.5 95.597.1 3 13.9 93.8 63.9 95.6 97.6 4 13.8 93.3 63.9 95.6 97.3 5 13.8 92.764.2 95.8 97.3 6 13.7 92.3 64.4 95.9 97.8 7 13.6 92.0 64.1 95.9 97.8 813.5 91.7 63.9 96.0 97.9 9 13.4 91.5 64.0 96.0 98.9 10 13.4 91.2 63.996.0 97.9 11 13.4 90.9 64.1 96.1 97.9 12 13.4 90.7 64.1 96.0 97.8 1313.2 90.6 63.3 96.1 97.9 Reaction stopped, catalyst exposed to oxygenenviron, feed reintroduced 15 14.1 93.3 65.2 93.6 97.6 16 13.9 91.6 65.693.9 97.2 17 14.0 90.9 66.5 94.1 97.1 18 13.9 90.6 66.5 94.1 97.3 1913.7 90.4 65.6 94.2 97.9 20 13.6 90.2 65.6 94.3 97.6 21 13.6 89.7 65.994.3 97.7 21 13.6 89.8 66.0 94.4 97.6 22 13.5 89.5 65.6 94.5 97.5 2313.5 89.4 65.6 94.4 97.6 24 13.4 89.4 65.5 94.5 98.0 25 13.4 89.4 65.494.6 98.0 26 13.6 89.4 65.8 94.6 97.4 27 13.5 89.3 65.8 94.6 97.9 2813.6 89.2 65.9 94.6 97.6 29 13.5 89.1 65.6 94.7 97.5 30 13.4 89.0 65.594.7 97.9 31 13.3 89.0 65.1 94.7 98.0 32 13.3 89.0 64.8 94.8 97.5 3313.4 89.0 64.9 94.8 97.5 34 13.4 88.9 65.5 94.8 97.9 35 13.2 88.6 64.795.1 97.7 36 12.9 87.2 64.5 95.5 98.1 37 13.9 90.6 66.5 94.1 97.3Reaction stopped, catalyst exposed to oxygen environ feed reintroduced40 13.8 89.8 66.5 93.0 97.6 41 13.8 89.4 66.6 93.0 97.1 42 13.8 88.866.9 93.1 96.7 43 13.7 88.7 66.7 93.2 97.0 44 13.7 88.6 66.8 93.3 97.245 13.8 88.5 66.8 93.4 96.5 46 13.9 88.5 67.3 93.4 96.5 47 13.8 88.567.1 93.5 96.7 48 13.7 88.3 66.9 93.5 96.9 49 13.7 88.2 67.0 93.6 97.150 13.7 88.3 67.0 93.6 96.8 51 13.7 88.2 67.0 93.7 97.1 52 13.6 88.466.9 93.7 97.2 53 13.8 88.3 67.3 93.8 96.6 54 13.8 88.3 67.3 93.8 96.755 13.9 88.4 67.8 93.8 96.7 56 13.7 88.0 67.0 93.9 96.8 57 13.7 88.067.3 93.9 96.8 Reaction stopped, catalyst exposed to oxygen environ,feed reintroduced 59 13.8 91.1 65.8 92.3 97.4 60 13.9 90.1 66.6 92.496.6 61 14.0 89.6 67.4 92.5 96.7 62 14.0 89.4 67.4 92.5 96.7 63 13.989.3 67.2 92.6 96.9 64 14.0 89.2 67.5 92.6 96.7 65 13.9 89.2 67.3 92.796.9 66 14.1 89.7 67.3 92.7 96.7 67 14.1 89.7 67.3 92.7 96.8 68 14.089.1 67.4 92.8 96.5 69 14.0 89.0 67.7 92.8 96.8 70 13.9 89.0 67.4 92.997.0 71 14.0 89.1 67.3 92.9 96.6 72 13.9 89.0 67.0 93.0 97.0 73 13.989.0 66.9 93.0 97.0 74 14.0 89.0 67.5 93.0 96.8 75 13.8 88.5 66.8 93.196.8 76 13.9 88.3 67.3 93.2 96.7

TABLE 5 Deactivation Rate Catalyst Run Days ( mole %/day Fresh  2-13−0.0524 Pretreated 15-37 −0.0351 Regenerated 40-57 −0.0081 Regenerated59-76 −0.0013

Example 4 Catalyst Pretreatment

Extruded Catalyst B was used in a toluene methylation unit having threereactors in series (see FIG. 1). Each of the reactors containedapproximately 40 g of catalysts and operated at conditions (see Table 6)to convert toluene and methanol to produce xylene products. Thecatalytic test was conducted for 90 days during which catalysts in thethree reactors were pretreated as follows:

-   -   1^(st) reactor catalyst—pretreated after 76 days of run.    -   2^(nd) reactor catalyst—pretreated after 48 days of run    -   3^(rd) reactor catalyst—pretreated after 33 and regenerated        after 67 days of run

The catalyst showed a decline in activity, i.e., a decrease in tolueneconversion with time on stream. The deactivation rates for fresh load ofcatalyst in the 1^(st), 2^(nd) and 3^(rd) reactors were −0.025, −0.046and −0.071 mole %/day, respectively. The catalyst was pretreated byremoving coke by contacting with 5.2% O₂ with the balance being N₂ at atotal flow of 1.44 L/min and at a temperature of 550-570° C. for 19hours. The catalyst showed significantly decreased rates of deactivationafter pretreatment (see Table 7). For example, deactivation rate forfresh load catalyst in the 3^(rd) reactor was −0.071 mole %/day butafter the pretreatment the deactivation rates decreased to −0.058 andafter regeneration decreased to −0.026 mole %/day. The results showimproved catalyst stability after pretreatment and subsequentregeneration. The results are summarized in Table 7.

TABLE 6 1^(st) Reactor 2^(nd) Reactor 3^(rd) Reactor Total Days onStream 90 90 90 Catalyst Loading, g 40 40 40 120 Reactor Inlet Pressure,psig 21.7 20.7 19.8 Max Temp in Catalyst Bed, ° C. 530 540 550 FeedComposition^(a) Toluene, g/h 89.9 89.9 Methanol, g/h 7.0 5.9 5.0 17.9Hydrogen, g/h 17.4 17.4 Steam, g/h 18.2 18.2 WHSV, ^(b) h⁻¹ 2.4 2.0 1.7LHSV, ^(c) h⁻¹ 2.0 1.6 1.4 Toluene Conversion, mole % 15.7 15.6 14.339.0 Methanol Conversion, mole % 98.3 97.6 97.4 98.1 Mole % SelectivityToluene to Mixed-Xylenes 95.9 92.9 90.8 93.5 Toluene to p-Xylene 89.784.4 80.3 82.6 Methanol to Mixed-Xylene 68.1 65.5 57.9 64.4 ^(a)Feedstreams for 2^(nd) and 3^(rd) reactors contain unconverted tolueneand products such xylenes, C9+ aromatics, olefins, etc. Make-up methanolis added to the 2^(nd) and 3^(rd) reactors. ^(b) Based on toluene andmethanol feed

TABLE 7 Fresh Catalyst Regeneration* Catalyst in DeactivationPretreatment* Deactivation Reactor Rate Deactivation Rate Rate 1^(st)Reactor −0.025 −0.003 2^(nd) Reactor −0.046 −0.021 3^(rd) Reactor −0.071−0.058 −0.026 Deactivation rate is defined as the decrease in tolueneconversion (mole %/day) *1st reactor catalyst- pretreated after 76 daysof run. 2nd reactor catalyst- pretreated after 48 days of run 3rdreactor catalyst- pretreated after 33 and regenerated after 67 days ofrun

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

1. A process for the alkylation of aromatics comprising: pretreating aphosphorus-modified zeolite catalyst by contacting the catalyst withalkylation process reactants for at least two hours at processconditions to produce an alkylated aromatic product, and subsequentlycontacting the catalyst with a gaseous stream comprising oxygen at atemperature and for a time until there is no oxygen consumption;contacting the pretreated catalyst with alkylation process reactants atprocess conditions to produce an alkylated aromatic product; andrecovering the alkylated aromatic product.
 2. The process of claim 1,wherein the phosphorus-modified zeolite catalyst comprises a medium porezeolite having a 10 and/or 12 member ring channels system.
 3. Theprocess of claim 1, wherein the phosphorus-modified zeolite catalystcomprises a zeolite selected from the group consisting of ZSM-4, ZSM-11,ZSM-12, ZSM-22, ZSM-23, Zeolite Beta, Mordenite, MCM-22, andcombinations and mixtures thereof.
 4. The process of claim 1, whereinthe phosphorus-modified zeolite catalyst comprises a zeolite selectedfrom the group consisting of silica-alumina phosphates (SAPO), aluminumphosphates (AlPO) and combinations and mixtures thereof.
 5. The processof claim 1 wherein the phosphorus-modified zeolite catalyst containsphosphorus in an amount of from about 0.01 to about 0.15 g P/g zeolite.6. The process of claim 1 wherein the phosphorus-modified catalystexhibits at least two ³¹P MAS NMR peaks having maxima at from about 0ppm to about −55 ppm.
 7. The process of claim 1 wherein the contactingthe catalyst with the gaseous stream comprising oxygen at a temperatureof about 450° C. to about 700° C. for at least two hours.
 8. The processof claim 1 further comprising: d) regenerating the catalyst; e)contacting the regenerated catalyst with a feedstock comprising tolueneand methanol at process conditions to produce an alkylated aromaticproduct; and f) repeating steps d) and e).
 9. The process of claim 8,wherein the catalyst is regenerated by contacting the catalyst with anoxygen-containing gas until no oxygen consumption is detected.
 10. Theprocess of claim 1, wherein the alkylation process reactants are tolueneand methanol.
 11. The process of claim 10, additionally comprisingcofeeds with the alkylation process reactants comprising hydrogen andsteam.
 12. The process of claim 1, wherein the catalyst has an initialdeactivation rate before step (a), and a second deactivation rate afterstep (b), and wherein the initial deactivation rate is greater than thesecond deactivation rate.
 13. The process of claim 1, wherein thealkylation process reactants comprise alkyl-benzene.
 14. A process forpretreating a phosphorus-modified zeolite catalyst comprising contactingthe catalyst with alkylation process reactants for at least two hours atprocess conditions to produce an alkylated aromatic product and thencontacting the catalyst with a gaseous stream containing oxygen at atemperature and for a time until there is no oxygen consumption.
 15. Theprocess of claim 13, wherein the phosphorus-modified zeolite catalystcomprises a zeolite selected from the group consisting of ZSM-4, ZSM-11,ZSM-12, ZSM-22, ZSM-23, Zeolite Beta, Mordenite, MCM-22, andcombinations and mixtures thereof.
 16. The process of claim 13, whereinthe phosphorus-modified zeolite catalyst contains phosphorus in anamount of from about 0.01 to about 0.15 g P/g zeolite.
 17. The processof claim 13, wherein the phosphorus-modified catalyst exhibits at leasttwo ³¹P MAS NMR peaks having maxima at from about 0 ppm to about −55ppm.
 18. The process of claim 13, wherein the catalyst is contacted withoxygen at a temperature of about 450° C. to about 700° C. for at leasttwo hours.
 19. A pretreated catalyst comprising a phosphorus-modifiedzeolite which has been contacted first with alkylation process reactantsfor at least two hours at process conditions to produce an alkylatedaromatic product and second by a gaseous stream containing oxygen at atemperature and for a time until there is no oxygen consumption.
 20. Thecatalyst of claim 18, wherein the catalyst is a phosphorus-modifiedZSM-5 catalyst, wherein the zeolite has a silica:alumina molar ratio ofat least about 25, and wherein the phosphorus-modified zeolite catalystcontains phosphorus in an amount of from about 0.01 to about 0.15 g P/gzeolite, and exhibits at least two ³¹P MAS NMR peaks having maxima atfrom about 0 ppm to about −55 ppm.